1. Field of the Invention
The invention relates to time division multiplexed switching systems.
2. Prior Art
Time division multiplexed switching systems, including those which operate under the control of digital computers, are very well-known and widely used. For a discussion of time division multiplexed systems, see U.S. Pat. Nos. 3,115,552; 3,172,956; and 3,401,235.
In some cases, time division multiplexing of telephone signals is incorporated into switching devices such as private branch exchanges (PBXs). These devices interconnect with all classes of trunk lines, station lines and other lines to switch between lines and to concentrate signals on these lines to trunk lines connected to the public network. This permits efficient utilization of these public network access trunk lines. Numerous commercial systems are available, including those which employ redundant computers to provide high reliability in such telephone switches. One such time division multiplexed switching device shall be described in conjunction with FIG. 6. Typically, these devices include, as will be described, an interface unit (comprising answering supervision, analog-to-digital and digital-to-analog conversion), concentration unit, and switching means. Hereinafter for purposes of this application, these types of devices and related devices shall be referred to as "modules" or "switching modules".
To expand the capacity of these modules, particularly PBXs, switching and concentrating units are interconnected most often in a multi-stage hierarchical architecture. Two such prior art interconnecting schemes are discussed in conjunction with FIGS. 2 and 3. This hierarchical architecture is not always efficient. The single unit used in these systems to provide higher order switching must be large enough to handle some maximum number of modules. This higher order switching is not cost-effective when used with less than this maximum number of modules. The corollary to this is that these systems are not readily expandable beyond the maximum number of modules.
Graded multiple architecture has been used to interconnect lines using modular step-by-step switches in central offices. In such architecture, it is well-recognized that direct links provide efficient coupling for deterministic first offered traffic. Telephony traffic which overflows these direct links represents more random, fluctuating traffic. This overflow traffic is more efficiently handled on shared links (denoted here as a "bus"). Often a combination of a bus for handling overflows along with direct links are employed in distributed architecture. One such prior art system which has been in use many years is shown in FIG. 4.
Another architecture used in data networks employs a plurality of TDM switches in a ring configuration. In this configuration, calls may be routed around the ring through other modules to a particular module. However, if time slots are used in each of the intermediate modules through which the call passes, this results in relatively inefficient use of the TDM switch capacity in each module. Furthermore, if the ring is broken (by the failure of one module), then the remaining operable modules in the ring cannot be used to provide a two-way connection. This architecture is shown to some extent in FIG. 1b.
As will be seen, the present invention discloses an interconnecting architecture of the distributed type which combines characteristics of both the ring and graded multiple architectures. The described architecture is well-suited for interconnecting a plurality of commercially available digital TDM switches or other modules for use in telephony applications. Separate digital intertie hardware is employed which permits calls to be effectively routed through each of the modules without requiring the use of a time slot from within the min TDM bus of each module. Thus, the invention provides a dynamic form of the graded multiple architecture.